I'm using Xioami's MI 9 SE phone.

I have several PDF viewer apps installed: Mi Doc Viewer, Pdf Viewer Plus, MuPDF viewer...

However - whenever I try to open a PDF with one of these, or even with a file manipulation app, from a browser - I either get some error message (e.g. lack of permissions which I can't change, or some blurb in Chinese) - or just an empty screen.

What might cause this problem, and how can I circumvent or fix it>?